There is an increasing research interest in interfacing microelectrodes with tissues for applications ranging from neural signal sensing and stimulation in BCI (brain-computer interface) for sensorimotor control to chronic pain management, and deep brain stimulation (DBS), among others. In the specific area of neural probes, for example, substantial amount of research effort over the past several years had concentrated in advances in single and array of neural electrodes capable of recording signals from the brain as well as stimulating specific brain regions. These research efforts vary from those concentrating on stimulation and recording from a single neuron to complex networks of neurons using multiple electrodes in cortical and sensory areas in brains. For such probes, not only the architectures of the electrode structures, but also material properties are of major importance. Predictably, therefore, conventional and well-known materials that exhibit biocompatibility have been the traditional materials of choice. For instance, initial effort in neural probes employed glass tubes filled with electrolytes, where the glass acted as a biocompatible insulator. Later developments involved single and arrayed metal wire electrodes that were manually assembled in an array format.
MEMS (MicroElectroMechanical Systems) technology is interesting, because microfabrication of micron-sized electrodes arrays of variety of shapes using silicon, metal, and insulating and substrate materials, such as polyimide, have been made possible, thus spawning newer generation of neural electrodes. As a result, MEMS-based microelectrodes for neural recording and stimulation have, in general, enabled miniaturized and low-power high-density multisite arrays capable of interrogating a wider area. Carbon-based MEMS, which are fabricated by treating a pre-patterned organic structure made from negative tone photoresist polymer precursor to high temperatures in an inert or reducing environment, are finding applications in an increasing suite of devices such as fuel cells, plastic solar cells, bio-fuel cells, electrodes for dielectrophoretic cell separation systems, etc.
Carbon is finding increasing interest in the micro- and nanofabrication research community as a material of choice, where its conductivity, electrochemical stability in ionic solutions, response to chemical treatments for surface property modifications, and biocompatibility make it an ideal choice for electrochemical applications. With the added research and clinical desires to interface electronics with the human body for neural sensing and stimulation purposes, carbon—as an engineering material—can address numerous human health conditions for the research and industrial communities.
One photoresist that is commonly used in carbon-based and glassy carbon-based MEMS is SU-8, which is a high transparency, epoxy-based negative photoresist that enables creation of high-aspect-ratio structures using traditional UV photolithography. It derives its name from the presence of eight epoxy groups. The main advantages offered by glassy carbon material are excellent electrochemical stability, good electrical conductivity (low impedance), and excellent response to chemical surface treatments for surface property modifications. In addition, glassy carbon offers the ability to vary environmental conditions over a larger current-level, and hence pH, ionic concentration and temperature changes, making it an ideal choice for in-vivo bio-signal recording and stimulation. Moreover, patternable glassy carbon offers better and matched coupling with the tissues for use in bioelectronics due to its tailorable electrical, mechanical and electrochemical properties.
Because of the ease of manufacturing and structural flexibility that they offer, there has also been an increase in interest in polymer-based microelectrodes, particularly for ECoG (electrocorticography) applications, EEG (electroencephalography) applications and EMG (electromyography) applications. Tsang et al. had reported a flexible multisite microelectrode array of several electrodes for insect flight biasing using neural simulation. Rubehn et al. reported a 252-channel epicortical ECoG electrode array made of platinum electrodes on polyimide foil substrate. Their array had independently addressable electrodes and is among the largest electrode array reported so far. A stretchable electrode array for non-invasive wearable applications was developed by Ma et al. who used PDMS (poly dimethylsiloxane) substrate along with metal electrodes. However, their array was limited to EMG (electromyography) and EEG (electroencephalography) with no reported application in neural probing at the cortex or brain.
For μECoG or microelectrocorticographical applications, electrode materials that possess long-term high-fidelity performance in recording electrical and electrochemical signals are sought. As implanted devices, electrodes face a dynamically changing harsh biological environment where pH and ionic concentration fluctuate, and physical arrangement of blood vessels and neurons, for instance, shift. On the other hand, from tissue point of view, long-term implantable electrodes pose mechanical and electrochemical strains that affect the structure and health of cells of interest. Therefore, the minimization and elimination of these chronic detrimental interactions between electrodes and tissue is a major research focus. For this, the minimization of these mismatches by using a new class of electrode materials that are amenable to material property tailorability as well as lithographic patterning, such as glassy carbon (GC) is of significant research interest.
Glassy carbon, unlike almost all other microelectrodes, offers not only the unique electrical and electrochemical advantages of carbon, but also the ability to be patterned and also be mounted on substrates other than silicon, such as polyimide, which is stretchable and flexible. As a result, C-MEMS electrodes carry a significant potential to be a very competitive platform in complete sensor and actuator systems. This potential for a wider use of glassy carbon MEMS electrodes is, however, hampered by difficulties such as (i) incompatibility of their high-temperature pyrolysis process with CMOS, (ii) incompatibility of glassy carbon bump pads with soldering to metal wires, and (iii) thermal mismatch between resin and silicon substrate that produces warping of traces. Further, these electrodes have almost universally been fabricated on rigid silicon and quartz wafers—and hence severely limiting their wide applications—mainly because of the high-temperature process of pyrolysis that is integral to the fabrication.
However, as further progress in robust long-term clinical application of bio-probes is pursued, one of the fundamental challenges encountered in recording electrical and electrochemical signals from an implantable probe is their long-term high-fidelity performance. As implanted devices, electrodes face a dynamically changing harsh biological environment where pH and ionic concentration fluctuate, and physical arrangement of blood vessels and neurons, for instance, shift. On the other hand, from tissue point of view, long-term implantable electrodes pose mechanical and electrochemical strains that affect the structure and health of cells of interest. Therefore, the minimization and elimination of these chronic detrimental interactions between electrodes and tissue is a major research focus. For this, the major barriers that need to be overcome are the mismatch in mechanical stiffness, hardness, electrical impedance, and electrochemical behavior of the electrodes with that of tissue, which cause damages and risks associated with long-term tissue responses to implanted electrodes.
To enable long-term implantable devices, electrodes must be able to change mechanical stiffness, hardness, electrical impedance, and electrochemical behavior to match that of tissue. By creating glassy carbon-based MEMS electrodes and microstructures with tunable properties, devices suitable for micro-electrocorticography (ECoG) arrays can be manufactured that minimize the mismatches in properties that pose a barrier to biocompatibility of long-term implants. Therefore, it would be ideal to develop a class of electrode materials that are amenable to material property tailorability as well as lithographic patterning. In addition, it would be ideal if the materials have excellent conductivity, electrochemical stability in ionic solutions and excellent response to chemical surface treatments for surface property modifications, while at the same time being corrosion resistant, even in typical oxidizing environments. Attractive materials provide or allow an ability to tailor its mechanical, electrical, and electrochemical properties by varying the pyrolysis conditions, such as maximum temperature, duration of pyrolysis, and ramp rate. Ideal microstructures will have a fairly straightforward or easier manufacturing process. Ideal materials and microstructures would have at least some of the following applications: neural signal sensing, brain-computer interfacing, chronic pain management, deep brain simulation, fuel cells, solar cells, biofuel cells or a combination thereof.